You are overloading the 30 amp. Turn water heater to Propane.
Verify 120 volt amperage for "small" refers.
Normal refers are LP/electric. 120 volt AC should hold with AC running.
Next check for loose connections in 120 volt AV power distribution panel. Make sure you check for melted wiring. Look at the neutrals.
Measure your voltage. Lower the voltage the higher the amperage.
Bud
USAF Retired
Pace Arrow
2003 Chev Ice Road Tracker